1. Field of the Invention
The present invention relates to measurement systems for monitoring various parameters. In particular the present invention relates to ultrasonic parameter measurement systems that operate by emitting and detecting acoustic waves to determine information representing the monitored parameters.
2. Background Art
Ultrasonic instruments have been used to monitor the parameters of materials in a large variety of measuring applications. When the level or height of the surface of a material from the bottom of a container is the parameter to be measured, an ultrasonic instrument can include a transducer for emitting an ultrasonic pulse in the direction of the material and detecting the echo of the ultrasonic pulse reflected from the surface of the material.
The time interval between the emission of the ultrasonic pulse and the detection of the echo of the ultrasonic pulse is determined by the distance between the transducer and the surface of the material. The time interval is measured by the ultrasonic instrument and the measured time is multiplied by the speed of sound to calculate the round trip distance traveled by the ultrasonic pulse. The product of the multiplication can be divided by two to yield the separation between the transducer and the surface of the material and otherwise scaled as desired.
In practice ultrasonic measurement can be complicated by many factors. For example, the speed of sound through a medium is not a constant. The speed of sound varies with the temperature and the composition of the medium through which the ultrasonic pulse travels. Compensating for factors that alter the speed of sound is well known. However, even if the factors altering the speed of sound are not compensated, the effects of the factors are predictable and an uncompensated measurement can be considered approximately correct.
Additionally, it can be difficult to determine when an echo of a transmitted ultrasonic pulse is received by an ultrasonic measurement instrument. Received signals interpreted as a reflected pulse from the material surface by an instrument can be caused by something else in the measuring environment. This can lead to an instrument output that is in error. Furthermore, a received signal can include echo signals returned by a variety of paths. The timing and strength of the various received signals depend upon the path traveled and the materials encountered along the path.
For instance, a received signal can contain an echo from the material surface, or an echo from the bottom of the vessel containing no material, and an echo from an object inside the vessel, such as a pipe. When an ultrasonic measurement instrument receives a strong echo from, for example, a pipe located above the material surface, it can generate an output indicating that the material level is at the level of the pipe. The effects of errors of this nature can be serious. For example, when hazardous materials are involved a critical control action can be undertaken or not undertaken in reliance upon an incorrect indication of material condition.
One method of addressing some of the problems associated with ultrasonic instrument measurement is profiling. In the profiling process, the vessel containing the material to be measured is emptied. Ultrasonic pulses are emitted into the vessel and the echo signals are received. All echo signals received are stored. The stored information can later be used to cancel the spurious echoes.
While the profiling measurement method can improve measurement accuracy, it has several drawbacks. There is substantial expense associated with the electronic systems necessary to acquire, store and compare the data. Furthermore, profiling is cumbersome and time consuming. It is also subject to errors from changes in the measuring environment, such as changes in the material properties or modification of the vessel or its internal apparatus. Any such changes may require that the profiling be repeated.
It is therefore a general object of the present invention to provide an ultrasonic measuring system that avoids the drawbacks of existing ultrasonic measuring systems.
It is another object of the present invention to provide an ultrasonic measuring system that is easily calibrated.
It is another object of the present invention to provide an ultrasonic measuring system with improved measurement accuracy.
It is another object of the present invention to provide an ultrasonic measuring system that is adaptable for use in standard instrumentation signaling and power systems, including two-wire systems and digital signaling systems.
It is another object of the present invention to provide an ultrasonic measurement system which is simple, rugged, reliable and inexpensive.
In accordance with the foregoing objects, the present invention includes a novel system for processing received ultrasonic signals that improves the accuracy of determining when a received signal corresponds to a material condition of interest. The system of the present invention includes means for varying the responsiveness of the received signal processing circuitry as a function of the elapsed time after an ultrasonic pulse is transmitted.
Other objects and features of the present invention will be understood with reference to the drawings, the following description and the appended claims.